Just come on your passport for 30 days go up to immigration and they will extend it for you for 1960baht. Or you could do the Cambodian run and come back in for another 30 days
When exiting Thailand you use your Thai passport. Then when you get the US use you U.S. passport. When leaving the US you use the US passport. When entering Thailand you use your Thai passport.
It's just been announced on Thai TV that you only have to quarantine for 7 days starting in April . If you've been vaccinated. if you have not been vaccinated then it is 10 days.
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